I got to know about Zato recently from the main developer, Dariusz Suchojad, who had earlier written to me regarding my blog post about PyMQI:

PyMQI, Python interface to IBM WebSphereMQ (formerly IBM MQSeries):

http://jugad2.blogspot.com/2013/02/pymqi-python-interface-to-ibm.html

Dariusz was a maintainer of PyMQI and also a developer on Spring Python, which is sort of a port of the Java Spring framework to Python.

Zato docs (quite detailed):

https://zato.io/docs/index.html

Part 1 of a basic Zato tutorial:

https://zato.io/docs/tutorial/01.html

I took a look at the tutorial. Broadly, it shows how to install Zato, create a simple Zato service in Python, that talks to PostgreSQL and Redis, and deploy it. Two servers get created, behind a load-balancer, and the service gets hot-deployed to the servers. Then curl is used to access the service. (This tutorial does not create a real client; curl is used to simulate one.)

Zato looks interesting and powerful (and somewhat complex, but that is to be expected for a product like an ESB).